1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to techniques for closing a drift between adjacent in situ oil shale retorts to minimize gas flow through such drifts during retorting operations.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semi-arid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer; oil shale is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; 4,043,598; and 4,192,554, which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with the hot carbonaceous material to produce heat, combustion gas, and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in, or added to, the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
In carrying out retorting operations, it can be desirable to isolate adjacent in situ retorts from one another so that operations in one retort do not affect operations in adjacent retorts.
When forming a group or cluster of in situ retorts, vertical partitions or barriers of unfragmented formation are left between adjacent fragmented masses in the group of retorts. The partitions or barriers between individual retorts serve as gas barriers which isolate adjacent retorts, making it possible to independently control retorting operations in each fragmented mass within the group of retorts. The gas barriers also inhibit leakage of off gas into adjacent underground workings where operating personnel may be present.
When preparing a system of in situ oil shale retorts, it can be desirable to excavate retort development drifts through unfragmented formation between adjacent retort sites. Such drifting between adjacent retorts can be useful to provide access to excavate void spaces at different levels within each retort site, to drill blasting holes downwardly into the retort volume, and otherwise to assist in preparation of the retorts. It is also desirable to minimize gas flow through such development drifts interconnecting adjacent retorts so that during retorting operations, there is minimal fluid communication and disruption of retorting operations in either of the retorts.
A gas seal can be provided in development drifts interconnecting retorts for isolating retorting operations in adjacent retorts from one another. U.S. Pat. No. 4,133,580 to French discloses a method of forming a gas-impermeable seal in a drift between adjacent retorts. The gas seal includes an impervious layer of concrete or impervious clay a foot or so thick closing off the drift.
There is a need to provide an effective and inexpensive method for inhibiting gas flow through a drift between adjacent retorts. In an oil shale tract containing a large number of retorts, the number of retort development drifts between adjacent retorts can be considerable. There can be a number of such drifts excavated on each of several levels along each of many rows of retorts within the oil shale tract. Owing to the large number of gas seals required for such a tract of oil shale retorts, the system for sealing between retorts should not only be reasonably inexpensive, but also should not require an inordinate amount of additional effort or time as the retort system is being developed.